Boards

Is knowing what sounds better than what when you're actually putting it together. There's a million possibilities, sometimes it's impossible to tell whether something you've added actually sounds better or whether you just think it does because it's something new that wasn't there before. It must be even harder for those who make electronic moozik. I suppose that's why producers exist.

Starting with the initial demo. I have a tendency to keep starting from scratch with songs. It's how I used to write essays. Bloody annoying. But anyways, I'll just listen to earlier versions of a song and can figure out if something I've added or taken away has affected the song. I always find the worst thing is when you listen to something through headphones, switch to speakers and realise the entire sound has changed.

when your guitar sounds fucking BRILL to your ears then utterly SHITE when recorded, and no amount of shifting mics, compression or post recording twiddling will recapture what your ears heard coming out of the amp...

I always found the best sound came from removing the ball thing at the end of my Shure SM58 mic and inserting it into the sound hole of my acoustic guitar. You have to be a bit careful of getting a booming low end, but it always sounded pretty good. obviously you can always adjust the eq afterwards.

Honestly though I find that getting an idea down for reference as quickly as possible is the most important thing. As long as you can hear the notes clearly, right?

"Hey, not to like, force it on you, but I want a set of ears to see what's missing from this track other than mine". Inciuating you think something is wrong means they won't go in with expectations too high.

when a guitar string snaps and whips into your eye, temporarily blinding you and as you stumble about you trip over your effects pedals, collapse on the floor and then a hi-hat lands on your head. And then a rabid puma somehow gets into your recording studio and starts to gnaw at your thighs.

I always find this. It's tough making music on your own, especially when you're as stubbornly independent as I am. There are upsides to recording as a solo artist, though (you don't have to make any comprimises with bandmates, and you don't have to arrange practises), but there are many downsides.
I often long for someone to bounce ideas off. I guess that's where the 'producer' comes in, but i always imagine those kind of 'bouncing' relationships to be more 'artist-artist' collaborations than simply a 'producer serving an artist' collaborations. Inevitably you end up comprimising on something.
As for your problems with adding things to your pieces and wondering if they're actually any good, I would highly recommend the following:
1) record it (just a demo or a basic layout in Cubase or whatever) and leave it for a few days, and come back to it. You'll see things in a different light
2) if you really want a second opinion, just ask someone about it. Preferably someone who is a musician themselves, or a musical friend/relative, just so they could add some constructive feedback if necessary. It helps talking to someone with even just basic knowledge of music.
3) Oblique Strategies. I've found it really does help, so thank Mr Eno for that one. Here's a free net version, there's plenty of others all over the interweb: http://tools.blackhat-seo.com/strategies/4) Finally, have faith in what you do, but remember that not everything works first time. Try try and try again.